1. Technical Field
The present invention relates to a hydraulic stepless transmission and more particularly to a technique of controlling swash plate angles of movable swash plates of a hydraulic pump and/or a hydraulic motor forming the hydraulic stepless transmission.
2. Background Art
Conventionally, there is a known structure of a hydraulic stepless transmission (hereafter abbreviated as “HST”) formed of a hydraulic pump and a hydraulic motor, in which a hydraulic servo mechanism controls tilting of movable swash plates of the hydraulic pump and the hydraulic motor to regulate capacities of variable displacement-type hydraulic pump and hydraulic motor. As a structure of the hydraulic servo mechanism, a mechanism (automotive control) in which a solenoid valve mounted to the hydraulic pump or the HST automatically tilts the movable swash plates in proportion to increase of a rotational speed of the hydraulic pump, a mechanism (manual servo control) in which a speed change operation lever mounted to an outer portion of the HST operates the movable swash plate of the hydraulic pump, and the like are known.
On the other hand, the HST having a mechanism (load control mechanism) for carrying out speed control in such manners as to reduce a traveling speed of a work vehicle when a load of an engine is a set value or greater, and to make the traveling speed a set speed when the load of the engine is the set value or smaller is known. To put it concretely, the movable swash plate of the HST (hydraulic pump) is shifted to a speed reducing side so that a work load does not exceed a predetermined value to thereby prevent stalling.
As the HST having such hydraulic servo mechanism and load control mechanism, there is an HST as disclosed in Patent Document 1, for example.
This HST includes a variable displacement-type hydraulic pump and two hydraulic motors operated by hydraulic oil of the hydraulic pump. At least one of the two hydraulic motors is a variable displacement-type hydraulic motor and an output combining mechanism for combining respective output rotations of the two hydraulic motors to result in a single output rotation is provided. With this structure, the output rotation of the variable displacement-type hydraulic motor is adjusted to substantially double output torque to increase the maximum torque in the HST.
The HST disclosed in the Patent Document 1 includes a hydraulic servo mechanism in which a speed changing valve switches a speed changing cylinder interlocked with a movable swash plate to tilt the movable swash plate in the hydraulic pump and the movable swash plate of the hydraulic motor can be tilted. With this structure, if load torque on the engine increases during work of the work vehicle at a slushy place or the like, the movable swash plate of the one hydraulic motor formed as the variable displacement-type motor is tilted to thereby increase low-speed torque to perform speed control of the work vehicle.
A prior-art structure of a hydraulic stepless transmission having a feedback circuit using a shuttle valve will be described.
FIG. 19 shows a hydraulic circuit of the prior-art hydraulic stepless transmission having the feedback circuit using the shuttle valve.
In the feedback circuit in the prior-art hydraulic stepless transmission, hydraulic oil is fed back to load control mechanisms 4, 4b and 104 as pressure in a main circuit connecting a hydraulic pump and a hydraulic motor via a shuttle valve 204c. 
Patent Document 1: Japanese Patent Application Laid-open Publication No. 2004-11769